The present invention relates to a process for crystallising an inorganic substance, using a bed of crystals, which is traversed by a solution supersaturated in the substance to be crystallised.
The "OSLO" crystalliser is an apparatus well known for crystallising inorganic substances (British Chemical Engineering, August 1971, Volume 16, No. 8, pages 681 to 685; The Chemical Engineer, July/August 1974, pages 443 to 445; British Patent GB-A-418,349). This known apparatus comprises a vertical cylindrical cell and a vertical tube which is disposed axially in the cell and ends in the immediate vicinity of the bottom of the latter; an annular vertical chamber is thus defined between the axial tube and the cylindrical wall of the cell. In the operation of this known apparatus, a bed of crystals in the annular chamber is used, through which a solution supersaturated in the substance which it is desired to crystallise is caused to flow (for example an aqueous supersaturated solution of sodium chloride). This solution is introduced into the apparatus via the axial tube, in such a way that it radially penetrates the annular chamber, close to the bottom thereof, and subjects the crystals of the bed to a common rotation, comprising an ascending translation along the wall of the cell and a descending translation along the axial tube.
In this known apparatus, the purpose is to produce regular crystalline grains of spherical shape, whose mean diameter should be controllable by an appropriate choice of the dimensions of the apparatus and of the operating conditions. In practice, however, this known apparatus is rather unsuitable for obtaining large spherical grains, particularly due to the fact of attrition generated within the bed of crystals and of the common rotation, to which these are subjected.
In the document EP-A-0,352,847 (SOLVAY & Cie), a process and equipment are described which eliminate the said drawbacks. In this process and this equipment, a fluidised-bed reactor is used and the solution supersaturated in the substance to be crystallised is caused to circulate upwards across a distributor in such a way that, above the latter, the fluidised bed of crystals is generated; the mother liquor from the crystallisation is collected above the bed and recycled to a point upstream of the bed, after it has been supersaturated to reconstitute the original supersaturated solution.
The invention perfects the process and equipment of document EP-A-0,352,847 by providing a means which reduces the risk of parasitic crystal formation below the fluidised bed.
Starting from this situation, the invention relates to a process for crystallising an inorganic substance in a fluidised bed of crystals, which is generated by subjecting a solution supersaturated in the substance to be crystallised to an ascending circulation across a distributor, located below the bed, of a fluidised-bed reactor, the mother liquor from the crystallisation being collected at the bed exit and being supersaturated to reconstitute the said supersaturated solution; according to the invention, a fraction of the supersaturated solution is taken off upstream of the distributor and recycled to a point downstream of the distributor, before the mother liquor is collected.
In the process according to the invention, the crystals of the bed serve as seeds for the crystallisation of the inorganic material by desupersaturation of the supersaturated solution. They are generally small regular crystals of the inorganic substance which it is intended to crystallise.
The level of supersaturation of the supersaturated solution depends on various parameters, in particular the nature of the inorganic material, on its temperature and on the possible presence of solid or dissolved impurities. In practice, everything else being equal, it is advantageous to establish a maximum supersaturation level, which must always be limited in order to avoid accidental crystallisations on the walls of the crystallisation equipment upstream of the crystal bed, and also primary and secondary seeding within the solution.
The solvent of the solution is not critical, and water is generally preferred.
The temperature of the supersaturated solution is not critical. Nevertheless, it has been observed in practice that the rate of crystal growth in the bed is higher, the higher the temperature of the solution. It is advisable, however, for the temperature of the solution to remain below its boiling point at the pressure prevailing in the crystallisation chamber. For example, in the case where the process is applied to the crystallisation of sodium chloride, aqueous sodium chloride solutions can advantageously be used which have a degree of supersaturation of between 0.3 and 0.5 g/kg at a temperature of between 50.degree. and 110.degree. C. The degree of supersaturation expresses the mass of excess inorganic material relative to the mass corresponding to saturation of the solution.
The bed of crystals is a fluidised bed in accordance with the generally accepted definition (Givaudon, Massot and Bensimon--"Precis de genie chimique [Chemical engineering principles]"--Volume 1--Berger-Levrault, Nancy--1960, pages 353 to 370). To fluidise the bed, the stream of the supersaturated solution is caused to pass through a distributor disposed underneath the bed of crystals, in accordance with the usual technique for fluidised-bed reactors. The distributor is a fundamental element of fluidised-bed reactors. Its function is to divide the stream of solution into preferably parallel and vertical thin jets and additionally to impose thereon a defined pressure drop, controlled as a function of the bed dimensions, of the nature of the particles forming the bed, and of the solution (Ind. Eng. Chem. Fundam.-- 1980--19--G. P. Agarwal et al.--"Fluid mechanical description of fluidized beds. Experimental investigation of convective instabilities in bounded beds"--pages 59 to 66; John H. Perry--Chemical Engineers' Handbook--4th Edition--1963--McGraw-Hill book company--pages 20.43 to 20.46). It can, for example, be a horizontal plate perforated by regularly distributed orifices, a grate or a horizontal lattice, or an assembly of vertical channels.
Downstream of the bed of crystals, the mother liquor from the crystallisation is collected. This is a solution saturated in the substance to be crystallised. It is treated for converting it into a supersaturated state and then recycled into the supersaturated solution upstream of the distributor. The means used for obtaining supersaturation of the mother liquor are not critical. The supersaturation of the mother liquor can, for example, be obtained by altering its temperature or by subjecting it to partial evaporation.
Details regarding the operation of the fluidised bed, the supersaturated solution, the recycling of the mother liquor and the regeneration of the supersaturated solution are described in the document EP-A-0,352,847 (SOLVAY & Cie).